The first magneto-optical study of the4A2g→2T2gabsorption,2Eg→4A2gand2T2g→2T1g,2Egluminescences of MoCl3-6in Cs2NaYCl6, as well as the absorption spectrum of the spin-allowed4T2gand4T1gligand field states, are reported. Magneto-optical methods have allowed difficulties in a previous assignment to be resolved. Coupling of almost all electronic origins to α1g,t2uand botht1uMoCl3-6internal modes, as well ast1ulattice modes, is established, uniquely, from the sign of the magnetic circular dichroism and magnetic circular polarization of luminescence spectra. In addition, coupling to at1uinternal mode of the host YCl3-6unit is observed. Moderate coupling to theegJahn-Teller mode occurs for the2T2g(U′),2T1g(U′) and2Eg(U′) origins, particularly the former, but is very weak or absent for the2T2g(E″) and2T1g(E′) origins. A comparison of the relative dipole strengths induced byt1uandt2uvibrations, as well as the sign of the MCPL for the2Eg→4A2gtransition, are consistent with the intensity for the4A2g→2Eg,2T1g,2T2gtransitions being borrowed primarily from the4T2gstate via spin-orbit coupling. The observed2T1gand2T2gspin-orbit splittings of 102 and 75cm-1, respectively, are reduced by approximately one third from those calculated, and this reduction is attributed to a Jahn-Teller effect.